Signal amplifying system



Dec. 8, 1936. v F. R. FARROW, JR 2,063,304

SIGNAL AMPLIFYING SYSTEM Filed Nov. 26, 1950 zzasouxw Z5 I NVENTORZ Freder/c-AEF arrow (/5,

BY A I I H/SATTORNEY1 QC. SOURCE Patented Dec. 8, 1936 UNITED STATES PATENT OFFICE SIGNAL AMPLIFYING SYSTEM tion of Delaware Application November 26, 1930, Serial No. 498,231

11 Claims.

My invention relates to signal-amplifying systerns and it has particular relation to volumecontrol devices therefor.

It has, previously, been proposed to provide signal-receiving and signal-amplifying systems with volume-control devices whereby the gain in said system is either kept constant or is reduced in proportion to an increase in the initial amplitude of an incoming signal. Automatic volume-control systems, of the type to which I refer, are advantageous in the event that signals from a distant station are subject to pronounced fading inasmuch as they tend to maintain the sound-output from the system substantially constant. However, an automatic gain control for local or distant stations, or to compensate for fading, does not altogether eliminate a fortissimo passage. In addition, the limits of the broadcast amplifier and of sound levels on land wire hookups are also such that a true artistic cresendo is impossible, as is the playing of percussions at high volume.

It is a well-known fact that the relative intensity, or loudness, levels that an orchestra is capable of producing is of the order of one million to one. This difference is noted, for example, between a pianissimo violin-passage and the full ensemble of the orchestra. Such enormous ratios of sound intensity cannot be satisfactorily broadcast through the use of heretofore known systems, nor can they be satisfactorily received or recorded. In a radio transmitter network cross-talk is one of the limiting factors; in recording, groove-pitch imposes a limit upon the amplitude of the recorded sounds; while in the reception of radio signals a loud passage, if not reduced to average intensity by well-known volume-control devices, tends to cause the thermionic tubes to overload.

It is, accordingly, an object of my invention to provide, in a signal-amplifying system, means for automatically controlling the gain therein in response to the initial amplitude of a signal.

Another object of my invention is to provide, in a signal-amplifying system, means that shall function to automatically correlate the gain therein to the amplitude of a signal impressed upon the input circuit thereof.

Still another object of my invention is to provide, in a signal-amplifying system, means for preventing strong signals from causing the amplifying tubes to overload.

The aforementioned objects and others related thereto I prefer to attain, first, by normally biasing the thermionic or electric discharge amplifier tubes comprised in a signal-amplifying system to a negative potential such that they will adequately handle signals of average amplitude and, second, by providing means whereby, when said signals exceed the average amplitude, the bias is rendered sufficiently less negative to increase the gain in the controlled tube.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof will best be understood from the following description of certain specific embodiments when read in connection with the accompanying drawing in which:

Fig. 1 is a diagrammatic view of a portion of a signal-amplifying system including a preferred embodiment of my invention,

Fig. 2 is a diagrammatic view of a portion of an amplifying system including a modified form of my invention, and

Fig. 3 is a diagrammatic view of a portion of an amplifying system including yet another modified form of my invention.

The audio-frequency amplifier illustrated in Fig. 1 of the drawing, comprises a first amplifier stage constituted by a plurality of thermionic or electric discharge amplifier devices I and 3, disposed in push-pull relation, having a common circuit including a plurality of resistors 5 and I and a common output circuit including a plurality of resistors 9 and H. The first stage is coupled to a second stage over two coupling condensers l3 and I5, the second stage also including a plurality of thermionic tubes l1 and I9 having an output circuit constituted by the primary winding 2| of an audio-frequency transformer 23. The source of filament potential for the amplifying stages (not shown) may be of any convenient type.

A source 25 of D. C. potential is provided. Across the output terminals of this source is connected a potential-dividing resistor 21, having a grounded negative end. The junction between the input resistors 5 and I is also grounded and the filaments of the tubes comprised in the first amplifying stage are connected by a conductor 29 to a point on the potential dividing resistor that is, normally, at a sufiicient positive potential above ground to maintain the grids of the tubes I and 3 negatively biased to the proper amount.

The tubes included in the second amplifying stage are self-biased, by reason of the connection of a resistor 3| from a point in the common filament circuit thereof to the junction between a plurality of grid leaks 33, which junction is grounded.

The output transformer 23 is provided with a plurality of secondary windings 35 and 31, the winding 35 being connected to a sound producing device (not shown) and the other winding being connected in series with the cathode of a thermionic control-tube 39. The control-tube is illustrated in the drawing as being of the threeelectrode type and the grid and, plate thereof are shown as being connected together to serve jointly as an anode. It is, of course, to be understood that a two-element Fleming valve or any other rectifying device of the hot cathode type may be substituted for the tube illustrated.

The grid and plate of the control-tube are connected to a point 4| on the potential divider which is positive with respect to the connection thereto of the cathode.

In the operation of the system, as described, the steady current flowing through the potential dividing resistor 21 causes a potential drop between the connection 29 thereto of the cathodes of the tubes l and 3 in the first amplifying stage and the negative terminal of the resistor. At the same time, in the event that a signal of average amplitude is being impressed upon the input terminals of the system, a potential will be developed across the secondary winding 31 which supplies cathode current for the control-tube. By proper adjustment of a resistor 43 in the cathode heating circuit of the control-tube, the cathode of the tube may be substantially deprived of potential during such time as a signal of normal amplitude is being handled. The resistor may, of course, be manually adjusted to control the sensitivity of the energy-transfer circuit. When, however, the signal increases in amplitude, as at the beginning of an orchestral climax, an increased potential is developed over the winding 31, the cathode of the control tube is heated to a greater extent.

The space current, accordingly, that thereupon flows through the control-tube robs the potential-divider of some of its current and the potential-drop between the cathodes and the grids of the tubes in the first stage is reduced.

The gain in the first stage, therefore, increases and the amplifier satisfactorily handles the increased signal without distortion.

I have found it expedient, normally, to so bias the grids of the tubes in the first amplifier stage that these tubes operate on the curved portion of the grid potential-plate current curve, but since the input voltage is relatively small and since the tubes are disposed in push-pull relation, an inappreciable amount of distortion is caused therey.

It is interesting to note at this point that, since increased signal amplitude results in a lessening of the negative bias applied to the first stage, the said stage, when the signal amplitude increases, operates on a steeper and straighter portion of the grid potential-plate current curve and, consequently, the distortion therein is reduced.

Keeping in mind the principal objects of my invention, it is apparent that it may also be applied to controlling the plate potential in an amplifier as well as to controlling the grid-biasing potentials. As illustrated in Fig. 2, wherein elements equivalent to those shown in Fig. 1 are similarly designated, the space current path in the volume-control tube 39 may be connected serially between the positive terminal of the D. C. source 25 and the junction between the output resistors 9 and II of the first stage. In such event, the plate current supplied to the tubes in the first stage flows first through the volumecontrol tube.

The plate-impedance of the latter tube is a function of the electron emission from its cathode and, since the cathode is energized proportionately to the signal amplitude, an increased signal causes a lowering of the impedance of the tube with a consequent application of increased plate potential to the tubes in the first amplifying stage. The tubes included in the first stage of Fig. 2, like those of the second stage, are self biased by means of a resistor 6| connected between the filament lead and ground. With the tubes so biased any increase in plate potential and current increases the grid bias for reasons obvious to those skilled in the art, consequently when the signal current in the output of tubes I1 and I9 increases, thereby effecting an increase of plate potential and current to tubes I and 3, the negative bias of tubes I and 3 likewise increases.

The grid potential is initially adjusted for operation of the tubes I and 3 at a point on their grid potential plate current curve such that the increased negative bias with signal provides distortionless operation. It should be understood that the characteristic curve changes with increased plate potential corresponding to increased biasing potential so that in effect the power handling ability of the tubes is increased without reducing the operating point sufiiciently near anode current cut-oil. to introduce distortion.

This modification therefore serves to illustrate the operation of an amplifier stage for increasing the power handling capacity thereof in response to increased signal strength whereas in the circuit of Figure 1 the anode potential is maintained constant while the biasing potential is reduced thereby providing increased gain in the stage with increased signal strength.

It also falls within the scope of my invention to utilize a triode 45 as a control tube as indicated in Fig. 3 of the drawing. In such event, the cathode of the triode is supplied with potential from an outside source (not shown) and the cathode and grid are coupled, respectively, to the grids of the tubes 11 and I9 constituting the second amplifying stage by means of coupling condensers 4! and 49. A conductor 5| also extends from the cathode of the control tube to the negative terminal of the potential divider 21 and the plate of the said tube is connected to a point on the divider closely adjacent to the positive terminal thereof.

A grid leak 53, a biasing battery 55, and a potentiometer 57 are provided for fixing the normal D. C. grid bias of the triode 45, which is that of cutoff.

In the operation of the system shown in Fig. 3, as the amplitude of the signal icreases above the average, by reason of an orchestral climax or the like, the normal D. C. bias applied to the triode 45, has superimposed upon it A. 0. thereby causing rectification to take place; permitting the space current therein to increase and to therefore rob the voltage divider 2? of its normal current. The lessened current through the voltage divider gives rise to a lessened potential drop between the conductor 25!, which connects the oathodes of the tubes in the first stage thereto, and the negative terminal thereof with the result that the gain in the first stage is increased.

In addition to applying my invention to the audio-frequency amplifying stages of a receiving or transmitting system, it may also with but slight modification be applied to the radio frequency stages.

In such event, the action of the control tube, in robbing the voltage divider of current may be utilized to cause either an increase or decrease in the positive potential of the screen-grid of a screen-grid amplifier tube with respect to its filament. Inasmuch as the connections required for applying my invention to the radio frequency stage are obvious from the above description, taken in connection with Fig. 1 of the drawing, no specific illustration is necessary.

My invention is of particular value when applied to the audio frequency stages of a receiving system, the radio frequency stages of which are so controlled that signal-fading is compensated therein. In such a system, the correct reproduction of orchestral selections, including crescendo passages, is assured even though the fading compensator tends to maintain the sound output from the system at an average value. This, of course, would necessitate a double control; one for fading, operating on the radio frequency stages, the other for orchestral climaxes, operating on the audio frequency stages.

It will accordingly be apparent, from a consideration of the foregoing description of certain modifications of my invention, that it offers ad vantages not heretofore obtained through volume-control devices of well-known types. Furthermore, a signal amplifying system comprising my invention permits of much more realistic reproduction of the original sounds than is obtainable through the use of amplifiers not provided with means for correlating the gain therein to the amplitude of the impressed signal.

It should also be clearly understood that, by the interposition of properly designed filters between the secondary winding 37, in Fig. l, and the control-tube 39, my system may be made to preferentially respond to frequencies of any desired range. For example, the snare or bass drums could be accentuated, or passages of any other general type could be brought out.

My invention is also applicable to the electrical reproduction of phonograph records and the like. In such records, groove pitch and groove angle necessitate that a crescendo passage be cut down when recorded. My invention does not relate to this recording but to the reproduction of music so recorded. Thus, though a good crescendo passage could not be perfectly executed in the wax, the reproduction of it could be built up through my invention equal to the original level.

While I have described What I deem to be the most desirable embodiments of my invention, it is obvious to those skilled in the art that many of the details may be varied without in any Way parting from the spirit thereof. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a signal amplifying system, an electric discharge amplifier device, a signal output circuit, a thermionic device for controlling the gain in said system, said second named device having an electron emissive cathode, means for transferring energy from said output circuit to said cathode,

and an anode supply circuit for said first named device serially connected with said second named device.

2. In a signal amplifying system, a signal output circuit, a thermionic device for controlling the gain in said system, said device having an electron emissive cathode, means for transferring signal energy from said output circuit to said cathode to heat it in response to variations in said signal energy, means for manually controlling the transfer of said signal energy, and a potential supply source for said amplifying system to which the anode circuit of said thermionic device is connected.

3. In a signal amplifying system, an amplifier tube having an anode circuit, a thermionic control tube having an anode circuit, a potential supply souree for said amplifying system, circuit means serially connecting the anode circuits in said tubes to terminal points on said potential source whereby said anode circuits are coupled thereby, and means for causing the output impedance of said control tube to vary inversely with the amplitude of a signal.

4. The combination with a thermionic signal amplifying system including a thermionic amplifier device, of means providing a source of operating potentials therefor, said means including a. potential divider resistor having supply connections with said device, a signal output circuit for said system, a thermionic rectifier device having a cathode circuit connected with said output circuit, whereby it is energized in response to signal currents, and having an anode circuit connected in parallel with at least a portion of said potential divider resistor whereby the gain of said amplifying system is controlled in response to variations in signal amplitude.

5. The combination with a thermionic amplifier including a plurality of electric discharge amplifier devices and a supply circuit for conveying operating potentials to certain of the electrodes of said thermionic amplifier devices, of means responsive to the amplitude of signals transmitted through said amplifier for controlling the gain in said amplifier, said means including a thermionic control device having a cathode heating circuit connected with the signal transmission channel in said amplifier to receive the signal output therefrom, and an output circuit for said thermionic control device connected with said potential supply circuit for controlling the potentials conveyed thereby in accordance with changes in the average amplitude of signals transmitted through said channel.

6. The combination with an electric discharge amplifier, of circuit means for supplying anode operating potential thereto directly, and means responsive to changes in the average amplitude of signals transmitted through said amplifier for controlling the value of said potential, said means including a thermionic control device having a cathode connected with the amplifier to receive heating current therefrom and having an anode circuit, and an impedance device connected between said anode circuit and the potential supply circuit means.

7. The combination with an audio. frequency amplifier including an electric discharge amplifier device having a control grid, an output anode and a cathode, and a circuit for supplying an operating potential to one of said electrodes, of an electric discharge device connected with said amplifier to receive through its cathode heating circuit a portion of the output signal energy therefrom, thereby to control the impedance of said last named device, and said last named device being connected with said supply circuit to increase the operating potential supplied thereby in a positive direction in response to signals of increased average amplitude.

8. In a signal amplifying system the combination with an electric discharge device having a control electrode, a cathode, and an output electrode, of means for supplying signals thereto, means for supplying an operating potential for heating the cathode thereof, and means responsive to increases in the signal level for changing the operating potential supplied to said cathode in a positive direction.

9. In an audio frequency amplifier, the combination with an electric discharge amplifier device there-for, of a circuit for supplying biasing potential to said amplifier device, and means in said circuit for varying said biasing potential to increase the gain of said amplifier in response 10. The combination with an audio frequency electric discharge amplifier, of circuit means for supplying anode potential thereto, and means responsive to an increase in the signal level transmitted through said amplifier for increasing said potential, said last named means including an electric discharge device having an anode and a cathode connected in said supply circuit to increase the anode potential supplied thereby in response to an increase in the cathode excitation, and circuit means for supplying excitation current to said cathode from the signal output of said amplifier.

11, In a signaling system, an electron discharge tube having a cathode, a grid and an anode, means for applying a bias potential to said grid negative with respect to said cathode and for supplying operating current to said anode, means for impressing signals upon said tube, means responsive to an increase in the average amplitude of said signals for increasing the current supplied to said anode, and a self-bias resistor connected with said cathode for increasing the negative bias potential applied to said grid in response to said increased anode current, and for increasing the power amplifying ability of said tube.

FREDERICK REEVES FARROW, JR. 

